1. Field of the Invention
The present invention relates to an optical waveguide device and a coherent light source for use in a field such as optical information processing or optical communication. The present invention also relates to an integrated unit and an optical pickup device including the optical waveguide device or the coherent light source.
2. Description of the Related Art
In the optical information processing field, a small-sized light source for emitting light having a short wavelength is required for achieving high-density optical disks and high-definition displays. There are know techniques for generating light having a short wavelength. An example of such a technique is a second harmonic generation (hereinafter referred to as xe2x80x9cSHGxe2x80x9d) technique using a semiconductor device and an optical waveguide wavelength conversion (Yamamoto et al., Optics Letters Vol. 16, No. 15, 1156 (1991)) device using quasi phase matching (hereinafter referred to as xe2x80x9cQPMxe2x80x9d).
FIG. 20 is a diagram schematically showing a structure of a SHG blue laser 2000 using an optical waveguide type wavelength conversion device 83. Herein, a tunable semiconductor laser 80 having a distribution Bragg reflection (hereinafter referred to as xe2x80x9cDBRxe2x80x9d: Distributed Bragg reflector) region is employed. A tunable semiconductor laser having a DBR region is hereinafter referred to as a tunable DBR semiconductor laser. The tunable DBR semiconductor laser 80 is an AlGaAs DBR semiconductor laser whose input is in a power range of around 100 mW and whose oscillated wavelength is in a band range of around 0.85 xcexcm. The tunable DBR semiconductor laser 80 includes an active region 81 and a DBR region 82. The oscillation wavelength of the semiconductor laser 80 can be adjusted by changing an amount of current injected into the DBR region 82. The optical waveguide type wavelength conversion device 83 includes an optical waveguide 84 and a periodic polarization inversion region 85 on an X-plate MgO-doped LiNbO3 substrate (the X axis of crystal is substantially perpendicular to the substrate). Laser light emitted from a facet of the tunable DBR semiconductor laser 80 is coupled with the optical waveguide 84 of the optical waveguide type wavelength conversion device 83. With the configuration of FIG. 20, about 60 mW laser light out of about 100 mW laser output is coupled with the optical waveguide 84. By controlling the amount of current injected into the DBR region 82 of the tunable DBR semiconductor laser 80, the oscillation wavelength is fixed to fall within a wavelength allowance range of phase matching of the optical waveguide type wavelength conversion device 83, resulting in about 10 mW blue light having a wavelength of about 425 nm.
Such as SHG blue laser may be used for an optical disk recording and reproduction apparatus. Referring to FIG. 21, an SHG blue laser 2000 is mounted in an optical pickup 2100. A module (the SHG blue laser 2000) emits blue light which is in turn collimated by a collimator lens 86 and transmitted through a polarizing beam splitter 87 (hereinafter referred to as xe2x80x9cPBSxe2x80x9d) and a quarter-wave plate 88. Thereafter, the blue light is bent into a 90xc2x0 angle (substantially perpendicular to the plane of the figure) by a raising mirror (not shown) and converged by an objective lens 89 onto an optical disk 95. The light is reflected by the optical disk 95 and is bent into a 90xc2x0 angle by the PBS 87. The light is guided into a photodetector 91 (hereinafter referred to as xe2x80x9cPDxe2x80x9d) by a detection lens system 90 including a detection lens and a cylindrical lens. The photodetector 91 performs signal detection. With the optical pickup 2100, a high-density optical disk of about 10 GB or more can be reproduced.
Uchida et al., the Spring Convention of the Institute of Electronics, Information and Communication Engineers of Japan, C3. 28, 1994 reports a technique in which a 45xc2x0 cut is made in an optical waveguide to form a total reflection surface so that output light is in a direction substantially perpendicular to a substrate (see FIG. 22). In this case, an optical waveguide 93 of glass is formed by conducting double ion exchange on a glass substrate 92. A 45xc2x0 cut 94 is provided in the optical waveguide 93 by microprocessing. The processing is performed using an appropriate blade. The sides of the cut 94 are polished at the time of cutting so that a reflection surface whose loss is as low as 0.3 dB can be obtained.
Recently, there is a demand for small and thin optical pickups as computers are being downsized. In order to achieve smaller and thinner optical pickups, it is important to downsize not only a light source of the optical pickup, but also the configuration of the optical pickup. In this case, it is also important to take measures against returning light, interference noise, or the like described below.
1) Small and thin optical pickup
In the configuration of the conventional optical pickup 2100 of FIG. 21, the optical axis of the module is parallel to the optical axis of the optical pickup 2100. Therefore, the optical pickup 2100 is longer in the optical axis direction than in the width. When a semiconductor laser chip is employed, such an element is about 1 mm or less long, which is not problematic. In contrast, the SHG blue laser 2000 includes the optical waveguide type tunable device and the semiconductor laser. This causes the module size to be as long as 10 mm. The optical pickup 2100 becomes extremely long. Further, in the configuration of FIG. 21, the light detection system (the detection lens, the cylindrical lens, and the PD) is separated from the other parts of the optical pickup, causing the optical pickup to be large.
2) Measures against returning light
In the module including the semiconductor laser and the optical waveguide device, light reflected off the light-exiting facet of the optical waveguide is returned to the semiconductor laser. This causes the semiconductor laser to be in a multi-longitudinal mode, whereby noise characteristics are impaired.
3) Reduction in interference noise
In the SHG blue laser 2000, blue light is obtained by converting the wavelength of the semiconductor laser light which is used as a fundamental wave. Therefore, even if part of reflected light from the outside is returned to the semiconductor laser, the returning light does not contribute to noise. As a result, the semiconductor laser can be operated in a single mode, thereby obtaining a low level of noise (about xe2x88x92140 dB/Hz or less). However, blue light is highly coherent, so that if a cavity structure is externally provided, the amplitude of the blue light varies due to interference as the cavity condition is changed. In the configuration of the optical pick 2100 of FIG. 21, the surface of an optical disk 95 and the light emitting facet of the optical waveguide type wavelength conversion device form a confocal optical system. Therefore, as the optical disk is rotated changing a cavity condition, the intensity of light received by the PD 91 varies, causing deterioration of a signal waveform upon reproduction of the optical disk 95.
According to one aspect of the present invention, an optical waveguide device comprises a substrate having first and second surfaces; and an optical waveguide provided on the first surface of the substrate, having a light-incoming facet and a facet inclined with respect to the optical waveguide. Guided light incident to the optical waveguide through the light-incoming facet is totally reflected off the inclined facet, and the guiding light is transmitted through the first or second surface of the substrate.
In one embodiment of this invention, the guided light is transmitted through the first surface of the substrate.
In one embodiment of this invention, the guided light is transmitted through the second surface of the substrate.
In one embodiment of this invention, the substrate is made of a nonlinear optical material, the guided light incident to the optical waveguide as a fundamental wave is wavelength converted into a second harmonic wave, and the second harmonic wave is transmitted.
In one embodiment of this invention, the second harmonic wave is transmitted through the second surface of the substrate.
In one embodiment of this invention, a thickness of the substrate is about 0.3 mm or more and about 1.0 mm or less.
In one embodiment of this invention, a non-reflection portion is provided on the first surface of the substrate in which the second harmonic wave is substantially not reflected off the non-reflection portion.
In one embodiment of this invention, the non-reflection portion is made of antireflection coating.
In one embodiment of this invention, the second harmonic wave is transmitted through the first surface of the substrate.
In one embodiment of this invention, a non-reflection portion is provided on the first surface of the substrate in which the second harmonic wave is substantially not reflected off the non-reflection portion.
In one embodiment of this invention, the non-reflection portion is made of antireflection coating.
In one embodiment of this invention, the an angle attained from intersection of the inclined facet of the optical waveguide and the optical waveguide is about 45xc2x11xc2x0.
In one embodiment of this invention, the second harmonic wave has a guide mode.
In one embodiment of this invention, a whole side of the substrate corresponding to the inclined facet of the optical waveguide is inclined with respect to the optical waveguide.
In one embodiment of this invention, the inclined facet of the optical waveguide is a cut provided in the vicinity of the first surface of the substrate, and the cut is substantially perpendicular to the optical waveguide.
In one embodiment of this invention, the inclined facet of the optical waveguide is a groove provided in the vicinity of the first surface of the substrate, and the groove is substantially perpendicular to the optical waveguide.
In one embodiment of this invention, a cladding layer is provided on the first surface of the substrate excluding at least the groove.
In one embodiment of this invention, a cladding layer is provided on the first surface of the substrate.
In one embodiment of this invention, a relationship given by:
sin (xcex8) greater than n1/n2
is satisfied where n1 is a refractive index of the cladding layer; n2 is an effective refractive index of the optical waveguide with respect to light guided in the optical waveguide; and xcex8 is an angle attained from intersection of the first surface of the substrate and a normal line to a side of the groove.
In one embodiment of this invention, the inclined facet of the optical waveguide is a side of a groove provided in the vicinity of the first surface of the substrate in which the groove is substantially perpendicular to the optical waveguide; and
a relationship given by:
t2 less than t less than t1
is satisfied where t1 is a depth of a guide mode of a fundamental wave guided in the optical waveguide; t2 is a depth of a guide mode of a second harmonic wave guided in the optical waveguide; and t is a depth of the groove.
In one embodiment of this invention, a cladding layer is further provided on the first surface of the substrate;
the inclined facet of the optical waveguide is a side of a groove provided in the vicinity of the first surface of the substrate in which the groove is substantially perpendicular to the optical waveguide; and
a relationship given by:
nc2/nf2 less than sin (xcex8) less than nc1/nf1
is satisfied where nc1 is a refractive index of the cladding layer with respect to the fundamental wave; nc2 is a refractive index of the cladding layer with respect to the harmonic wave; nf1 is a refractive index of the optical waveguide with respect to the fundamental wave; nf2 is a refractive index of the optical waveguide with respect to the harmonic wave; and xcex8 is an angle attained from intersection of the first surface of the substrate and a normal line to the side of the groove.
In one embodiment of this invention, a diffraction grating is provided on the second surface of the substrate.
In one embodiment of this invention, the substrate is made of a first birefringence optical crystal.
In one embodiment of this invention, the optical waveguide device further comprises a second birefringence optical crystal provided in an optical path of the transmitted light. An optical axis of the second birefringence optical crystal is orthogonal to an optical axis of the first birefringence optical crystal of the substrate.
In one embodiment of this invention, one of the first and second surfaces is substantially in a shape of a cylinder in which the guided light is transmitted through the one of the first and second surfaces.
In one embodiment of this invention, the optical waveguide device further comprises a cylindrical lens provided in an optical path of the guided light transmitted from the optical waveguide device.
In one embodiment of this invention, the optical waveguide device further comprises a concave package. The optical waveguide device is provided in the concave package; and the concave package is sealed by the second birefringence crystal.
In one embodiment of this invention, the optical waveguide device further comprises a concave package. The optical waveguide device is provided in the concave package; and the concave package is sealed by the cylindrical lens.
In one embodiment of this invention, the substrate has a first facet including the light-incoming facet of the optical waveguide and a second facet opposite the first facet; an angle attained from intersection of a plane including a line of intersection of the inclined facet and the first surface of the substrate and a line of intersection of the second facet and the second surface of the substrate, and a direction in which the guided light is transmitted, is greater than half a divergence angle of the guided light transmitted through the second surface of the substrate.
In one embodiment of this invention, a relationship given by:
xcex81/2 less than xcex8
is satisfied where xcex8 is an angle attained from intersection of a plane including a line of intersection of the inclined facet and the first surface of the substrate and a line of intersection of the second facet and the second surface of the substrate, and a direction in which the guided light is transmitted; and xcex81 is a divergence angle of the guided light output through the second surface of the substrate.
In one embodiment of this invention, the wideguide device further comprises a periodic polarization inversion region provided in the first surface of the substrate.
In one embodiment of this invention, the periodic polarization inversion region is not provided in the vicinity of the inclined facet of the optical waveguide.
According to another aspect of the present invention, a coherent light source comprises a semiconductor laser and the optical waveguide device of the present invention.
In one embodiment of this invention, the semiconductor laser is a tunable semiconductor laser.
In one embodiment of this invention, a distance between a light-exiting facet of the semiconductor laser and the light-incoming facet of the optical waveguide is about 0 xcexcm or more and about 10 xcexcm or less.
According to another aspect of the present invention, an integrated unit comprises a coherent light source including a semiconductor laser and the optical waveguide device of the present invention in which light from the semiconductor laser is incident to the optical waveguide; a photodetector for detecting light associated with the light transmitted from the optical waveguide device; and a submount in which the coherent light source and the photodetector are provided on the same surface of the submount.
In one embodiment of this invention, the integrated unit further comprises a light shielding member provided between the coherent light source and the photodetector.
In one embodiment of this invention, a height of a surface of the photodetector is higher than a surface on a light-exiting side of the coherent light source with reference to a surface of the submount.
In one embodiment of this invention, a concave portion is provided in a surface of the submount; and the coherent light source is provided in the concave portion.
According to another aspect of the present invention, an integrated unit comprises a coherent light source including a semiconductor laser and the optical waveguide device of the present invention in which light from the semiconductor laser is incident to the optical waveguide device; a photodetector for detecting light associated with the light transmitted from the optical waveguide device; and a submount having a first surface and a second surface opposite the first surface in which the coherent light source is provided on the first surface of the submount and the photodetector is provided on the second surface of the submount.
According to another aspect of the present invention, an integrated unit comprises the coherent light source of the present invention; a concave package in which the coherent light source is provided in the concave package; and a transparent substrate for sealing the concave package in which a diffraction grating is provided on a surface of the transparent substrate.
According to another aspect of the present invention, an optical pickup comprises the integrated unit of the present invention; and a focusing optical system for converging light transmitted from the integrated unit. A diffraction grating is provided on a surface of the optical waveguide device of the integrated unit, the guided light being transmitted through the surface.
According to another aspect of the present invention, an optical pickup comprises the integrated unit of the present invention; and a focusing optical system for converging light transmitted from the integrated unit.
In one embodiment of this invention, the photodetector of the integrated unit is divided into at least three regions including a first region positioned at a middle of the photodetector, a second and third regions on opposite sides of the first region; the second and third regions with respect to the first region are provided in a direction substantially perpendicular to a grate of the diffraction grating.
According to another aspect of the present invention, an optical pickup comprises a semiconductor laser; the optical waveguide device of the present invention in which light from the semiconductor laser is incident to the optical waveguide device; two photodetectors for detecting light associated with light transmitted from the the optical waveguide device; a submount in which the coherent and the two photodetectors are provided on the submount; a focusing optical system for converging the light transmitted from the optical waveguide device; and a diffraction element having lens action provided in the focusing optical system. The two photodetector are provided on opposite sides of the optical waveguide device; the two photodetector each have at least a center portion and a peripheral portion; and the two photodetector are irradiated with light diffracted by the diffraction element.
Hereinafter, functions of the present invention will be described.
According to the present invention, an optical waveguide device includes a substrate having a first and second surfaces, and an optical waveguide provided on the first surface of the substrate. Guided light is totally reflected off an inclined facet of the optical waveguide so that the reflected light can be output through the first or second surface. Therefore, in a coherent light source (module) of the present invention including the optical waveguide device and the semiconductor laser, light is transmitted in a direction toward the first or second surface which is different from the optical axis direction of the coherent light source. A small-sized optical pickup can be achieved. Light, which is reflected off the light-exiting facet of the optical waveguide and returned to the semiconductor laser as in the conventional optical waveguide device having a vertical light-exiting facet, is reduced, thereby improving noise characteristics.
When the thickness of the substrate included in the optical waveguide device is less than about 0.3 mm, it is difficult to handle the substrate. When the thickness of the substrate included in the optical waveguide device is more than about 1.0 mm, the astigmatism is large at the focusing light spot. Preferably, the thickness of the substrate is about 0.3 mm or more and about 1.0 mm or less.
When the substrate of the optical waveguide device is made of a nonlinear optical material, and a fundamental wave of the semiconductor laser incident to the optical waveguide of the optical waveguide device is wavelength converted to a second harmonic wave, light (second harmonic wave) externally reflected and returned to the semiconductor laser does not contribute to noise. However, blue light is highly coherent, for example. Therefore, when the second harmonic wave is transmitted through the second surface, an antireflection coating to the second harmonic wave may be preferably provided on the first surface so that the returning light can be prevented from being reflected off the first surface.
According to the present invention, guided light is totally reflected off the inclined facet of the optical waveguide so that the reflected light can be output through the first surface. Therefore, in a coherent light source (module) of the present invention including the optical waveguide device and the semiconductor laser, light is transmitted in a direction toward the first surface which is different from the optical axis direction of the coherent light source. A small-sized optical pickup can be achieved. Light, which is reflected off the light-exiting facet of the optical waveguide and returned to the semiconductor laser as in the conventional optical waveguide device having a vertical light-exiting facet, is reduced, thereby improving noise characteristics.
When the substrate of the optical waveguide deice is made of a nonlinear optical material, and a fundamental wave of the semiconductor laser incident to the optical waveguide of the optical waveguide device is wavelength converted to a second harmonic wave, light (second harmonic wave) externally reflected and returned to the semiconductor laser doe snot contribute to noise. However, in this case, light reaches the first surface immediately after being totally reflected off the inclined surface. Therefore, an antireflection coating to a fundamental wave may be preferably provided on the first surface so that the returning light can be prevented from being reflected off the first surface.
In the optical waveguide device, when the inclined facet of the optical waveguide is inclined at an angel other than about 45xc2x0 with respect to the optical waveguide, coma aberration occurs at a focusing light spot. Therefore, the inclined facet of the optical waveguide is preferably inclined at an angle of about 45xc2x0 (45xc2x11xc2x0) with respect to the optical waveguide.
Further, when the substrate of the optical waveguide device is made of a nonlinear optical material, and a fundamental wave of the semiconductor laser incident to the optical waveguide of the optical waveguide device is wavelength converted to a second harmonic wave, the harmonic wave is preferably in a guide mode. The thickness of the substrate can be smaller than in a radiation mode so that the astigmatism or the like can be reduced. Moreover, when light in the optical waveguide is in a radiation mode, the thickness of the substrate needs to be increased in order to be transmitted in the substrate direction. When a groove or the like is provided on the first surface so that an inclined facet can be provided, the groove needs to be deep. Taking into account optical disk recording and reproduction apparatus applications, it is difficult to obtain a satisfactory light focus characteristic in the case of the radiation mode.
In the optical waveguide device, a groove or cut may be provided in the vicinity of the first surface, substantially orthogonal to the optical waveguide. A portion (a side) of the groove or cut can be an inclined facet of the optical waveguide. In this case, such an inclined facet can be easily provided by etching or the like used in a semiconductor process, as compared with the optical polishing. Note that damages to the optical waveguide device can be reduced in handling it more significantly when a light-exiting portion of the optical waveguide is concave than when it is convex.
When the first surface of the substrate on which the optical waveguide is provided contact the submount, a propagation loss in the optical waveguide is significantly increased. A cladding layer is preferably provided on the first surface of the optical waveguide. In this case, when the cladding layer is provided on the groove of the substrate, the reflectance of the groove may be decreased depending on the refractive index of the substrate. The cladding layer is preferably provided on the first surface of the substrate excluding the groove.
When the cladding layer is provided on the first surface of the substrate, the side of the groove (the inclined facet of the optical waveguide) can have a total reflection property if a relationship given by:
sin (xcex8) greater than n1/n2
is satisfied where n1 is the refractive index of the cladding layer; n2 is the effective refractive index of the optical waveguide with respect to light guided in the optical waveguide and xcex8 is the angle attained from intersection of the first surface of the substrate and a normal line to the side (inclined facet) of the groove.
Further, when a relationship given by:
t2 less than t less than t1
is satisfied where t1 is the depth of a guide mode of the fundamental wave guided in the optical waveguide; t2 is the depth of a guide mode of the second harmonic wave guided in the optical waveguide; and t is the depth of the groove, the second harmonic wave is totally reflected off the groove while the fundamental wave passes through the groove, so that the harmonic wave and the fundamental wave can be separated. Thereby, an amount of the fundamental wave mixed in an amount of the harmonic wave is reduced. Further, the separated fundamental wave is detected so that an output characteristic of the fundamental wave can be monitored.
When a relationship given by:
nc2/nf2 less than  sin (xcex8) less than nc1/nf1 
is satisfied where nc1 is the refractive index of the cladding layer with respect to the fundamental wave; nc2 is the refractive index of the cladding layer with respect to the second harmonic wave; nf1 is the refractive index of the optical waveguide with respect to the fundamental wave; nf2 is the refractive index of the optical waveguide with respect to the second harmonic wave; and xcex8 is the angle attained from intersection of the first surface of the substrate and a normal line to the side of the groove, if the thickness of the cladding layer is appropriate, only the harmonic wave is totally reflected off the groove, thereby allowing wavelength separation. Therefore, an amount of the fundamental wave mixed in the harmonic wave can be reduced. Further, an output characteristic of the fundamental wave can be monitored by detecting the separated fundamental wave.
Further, when light is output through the second surface of the substrate opposite the first surface, a diffraction grating may be provided on the second surface so that the number of parts can be reduced.
When light is output through the second surface of the substrate included in the optical waveguide device, light reflected off the inclined facet is propagated in the substrate as a divergence beam, having various directional components. When the substrate is made of a birefringence optical crystal, the refractive index thereof with respect to ordinary light is different from the refractive index thereof with respect to extraordinary light. Therefore, light beams transmitted from the optical waveguide device has different phases between in a direction parallel to the optical waveguide and a direction substantially perpendicular that direction. This causes the wavefront of collimated light beams has astigmatism components. Therefore, a second birefringence optical crystal whose optical axis is orthogonal to that of a birefringence optical crystal included in the substrate is preferably provided in a divergence optical path of the transmitted light from the optical waveguide device so as to compensate for an astigmatism component.
Alternatively, the substrate surface (second surface) through which light is transmitted from the optical waveguide device may be substantially in the shape of a cylinder, thereby compensating for the astigmatism.
A cylindrical lens may be provided in the optical path of the transmitted light from the optical waveguide device, thereby compensating for the astigmatism.
In the coherent light source of the present invention, when the optical waveguide device and the semiconductor laser are provided in a concave package, the concave package may be sealed using the second birefringence crystal as a sealing plate, thereby reducing the number of parts and therefore reducing cost. The sealing plate may be the above-described cylindrical lens.
The distance between the light-exiting facet of the semiconductor laser and the light-incoming facet of the optical waveguide is about several micrometers (e.g., 0 xcexcm or more and 10 xcexcm or less) so that a direct coupling is performed without a coupling lens. In this case, returning light from the light-incoming facet is prevented, thereby reducing noise.
In the SHG element, the wavelength of light incident to the SHG element needs to be coincident with the phase matching wavelength of the SHG element. Therefore, using a tunable semiconductor laser, the oscillation wavelength of the semiconductor laser can be fixed in the phase matching wavelength intensity range of an optical waveguide type wavelength conversion device. Such a function can be integrated with a coherent light source, thereby downsizing and stabilizing the coherent light source.
In the integrated unit of the present invention, the coherent light source in which light is output through the second surface opposite the first surface on which the optical waveguide is provided, and the photodetectors are provided on the same surface of the submount. Thereby, the photodetectors can be integrated with the submount in the vicinity of the second surface through which light is output. Moreover, the photodetectors and the coherent light source can be easily and precisely aligned with each other, thereby efficiently assembling the integrated unit.
A light shielding member may be provided between the coherent light source and the photodetectors so that stray light is prevented from reaching the photodetectors. The height of the surfaces of the photodetectors may be higher than the surface on the light-exiting side of the coherent light source with reference to the surface of the submount. Alternatively, a concave portion may be provided in the submount surface so that the coherent light source is provided in the concave portion.
In the integrated unit of the present invention, the coherent light source, in which light is output through the first surface of the substrate on which the optical waveguide is provided, is provided on a first surface of the submount, while the photodetectors are provided on a second surface of the submount. Thereby, the stray light is prevented from reaching the photodetectors, without the above-described light shielding member or groove.
Further, when the integrated unit or the coherent light source is provided in the concave package, a diffraction element is used to obtain a stable optical system of an optical pickup. The concave package may be sealed with a transparent plate on which a diffraction grating is provided. Thereby, the number of parts can be reduced.
Using the integrated unit of the present invention, a small and thin optical pickup of the present invention can be achieved. A diffraction grating may be provided in the focusing optical system, a substrate surface of the optical waveguide device, or the transparent plate sealing the concave package. Thereby, the number of parts can be reduced. The transmitted light is split into three light beams of 0th order and xc2x11st orders by the diffraction grating. The 0th order light beam is applied to a target track on an optical disk. The reflected light from the optical disk is incident to the focusing photodetectors which in turn detects an RF signal. The xc2x11st order light is used as subbeams for detecting tracking error signals, and the subbeams are detected by the respective tracking photodetectors, thereby obtaining the tracking error signals from differential output signals of the subbeams.
Further, the photodetectors may be provided on opposite sides of the coherent light source. Each photodetector is divided into a central portion and a peripheral portion (focusing photodetectors). In addition, the focusing optical system and a second diffraction element (hologram) having lens action may be provided. In this case, when the light reflected off an optical disk is diffracted by the second diffraction element, a light spot before focus is created on one of the focusing photodetector while a light spot after focus is created on the other of the focusing photodetectors. Therefore, signal detection can be performed using a spot size detection (SSD) technique.
Thus, the invention described herein makes possible the advantages of providing an optical waveguide device, a coherent light source, an integrated unit, and an optical pickup device which are small and thin and in which returning light to a semiconductor laser and interference noise are prevented.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following description with reference to the accompanying figures.